Understand Cross-Origin Resource Sharing (CORS), the Same-Origin Policy, how browsers enforce origin boundaries, and how to safely enable cross-origin API requests.
Frequently Asked Questions
Q1. What is the Same-Origin Policy (SOP)?
The Same-Origin Policy is a fundamental browser security mechanism that prevents a malicious script on one website from reading sensitive data or DOM contents from another website without explicit permission.
Q2. Does CORS protect my backend server from malicious requests?
No. CORS is enforced by the client browser, not the server firewall. An attacker can easily send HTTP requests using cURL, Python, or Postman directly to your server without sending an Origin header or adhering to CORS.
Q3. What constitutes a different origin?
An origin consists of Protocol + Hostname + Port. For example, http://api.example.com and https://api.example.com are different origins (different protocol), as are https://example.com:3000 and https://example.com:8080 (different port).
Q4. Why does my API work in Postman or cURL but fail in React or Vue?
Postman and cURL are command-line/desktop HTTP clients that do not enforce the browser Same-Origin Policy. Browsers intercept the response and block JavaScript from reading it when CORS response headers are missing.
Q5. Can I fix CORS issues strictly in frontend client code?
Generally no. CORS headers must be sent by the server responding to the request. On the frontend, you can only set up a local development proxy (e.g., Vite proxy or Next.js rewrites) to avoid cross-origin requests during local development.